1. Field of the Invention
This invention relates to a process for producing a continuous discharge of coal products from a coal deashing system utilizing particular solvents while operating at elevated pressures and at temperatures near the critical temperature of the solvents.
2. Description of the Prior Art
Heretofore, coal has been mixed with certain solvents at elevated temperatures and pressures to produce mixtures of dissolved and undissolved coal. Various systems have been proposed for separating the dissolved coal from the undissolved coal.
In U.S. patent application Ser. No. 973,876 filed Dec. 28, 1978 and assigned to the same assignee as the present invention, an improved system is disclosed for effecting such separation.
In that system, a feed mixture comprising soluble coal products, solvent and insoluble coal products is separated in a first separation zone maintained at an elevated temperature and pressure into a first light fraction and a first heavy fraction comprising insoluble coal products and some solvent. The first heavy fraction also includes a small quantity of relatively low melting point soluble coal products such as coal tars that are dissolved in the solvent. The insoluble coal products comprise the undissolved coal, mineral matter, other solid inorganic particulate matter and other such matter which is insoluble in the solvent solution under the operating conditions of the process.
The first heavy fraction is withdrawn from the first separation zone and the pressure level is reduced at least about 100 psig. and preferably at least about 500 psig. to vaporize the solvent and yield the insoluble coal products in a relatively dry, powdery form referred to as ash concentrate. Upon the vaporization of the solvent from the first heavy phase, the formerly soluble coal tars and the like separate and collect upon the interior surface of the withdrawal conduit. Since this material is at a temperature near to or above its melting point, it exhibits a highly viscous behavior and can, upon contacting the relatively dry ash concentrate, cause the ash concentrate to adhere thereto and form an agglomerate which will block the withdrawal conduit. The formation of such a blockage requires termination of process operation to permit removal of the agglomerate. A tedious and expensive cleanout then is required to restore operation.
It would be desirable to provide a method by which the blockage problem associated with the withdrawal of the first heavy fraction from the first separation zone can be alleviated.